1. Field of the Invention
This invention relates in general to an exhaust gas purifying system for an internal combustion engine, and more particularly to a secondary air supply control system which controls the mixture of exhaust gas and secondary air with high accuracy so as to bring the gas mixture near the stoichiometric air fuel ratio.
2. Description of the Prior Art
In an internal combustion engine having an exhaust system provided with a catalytic device and a secondary air supply system positioned upstream thereof, it is desirable to control the secondary air fuel ratio to the theoretical ratio (excess air ratio .lambda.=1) in conformity with the operating condition. However, in the prior art system wherein secondary air supplied from its source is delivered, through the operation of an air by-pass valve responsive to the vacuum in the intake manifold, to a secondary air switching valve which is actuated by the manifold vacuum through a vacuum transmitting valve, there occur fluctuations in the amount of injected secondary air according to the position of the air switching valve so that difficulties have been encountered in accomplishing optimal secondary air compensation. Means have therefore been wanted to ensure sufficiently accurate secondary air supply to achieve the desired effect.
Particularly it is necessary under various operating conditions of the engine to obtain respective suitable rates of change in secondary air flow to maintain the secondary air fuel ratio in the exhaust system at the theoretical value (excess air ratio .lambda.=1).
In the prior art system, the speed of change in secondary air flow is varied by orifices, which however is fixed in diameter, thus posing a problem to solve in practicing suitable secondary air supply for all different engine running conditions.
Therefore, for the purpose of controlling the mixture of exhaust gas and secondary air with high accuracy so as to bring its excess air ratio near the optimal value (.lambda.=1) by decreasing or increasing the supplied amount of secondary air according to whether the excess air ratio detected by an O.sub.2 sensor provided in the exhaust system is larger or smaller than unity (.lambda.&gt;1 or .lambda.&lt;1) respectively, it is required to change the rate of operation or response of the secondary air switching valve according to the rate of change in inlet air flow.